1. Technical Field
The technology described below relates to a MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which make it possible to reduce influence by remnant magnetic fields due to eddy currents at the time of applying a pre-pulse for a desired object.
2. Description of Related Art
Magnetic Resonance Imaging is an imaging method which excites nuclear spin of an object set in a static magnetic field with an RF signal having the Larmor frequency magnetically and reconstructs an image based on NMR signals generated due to the excitation.
The fat suppression technique that suppresses MR signals from fat is known as one of contrast control techniques in the field of magnetic resonance imaging. Moreover, the technique that suppresses fat signals by applying a fat saturation pulse as a pre-pulse is known as one of fat suppression techniques. For example, there is a CHESS (chemical shift selective) technique that excites and saturates protons of fat by applying a 90° RF pulse that has a bandwidth matched with the resonance frequency of fat. Especially, in case of acquiring a MR image of a cross section of an object, the fat suppression technique that suppresses MR signals from fat as described above is essential since fat present in the section causes image degradation such as artifacts due to the chemical shift.
In addition, the technique that controls a contrast by applying a pre-pulse as described above is used for various purposes such as labeling as well as for fat suppression.
Meanwhile, at a rising edge and a falling edge of a gradient magnetic field applied in magnetic resonance imaging, an eddy current is induced on conductive parts of the MRI device due to a change in the gradient magnetic field. Accordingly, there is a problem that an artifact is generated in the image since a waveform of a gradient magnetic field to be applied is distorted due to a magnetic field generated by the induced eddy current.
Against this problem, a technique called pre-emphasis that outputs a gradient magnetic field with the addition of a gradient magnetic field having the polarity opposite to that of a gradient magnetic field generated by an eddy current in advance is known as a means to compensate for image degradation caused by an eddy current (see, for example, Japanese Publication of Patent Application No. 2001-258865). To be able to use the pre-emphasis, a time constant in attenuation and an intensity of an eddy current are necessary and these values are measured in advance. Note that, a time constant in attenuation and an intensity of an eddy current are specific to the MRI device.
However, the conventional compensation technique for an eddy current prevents generation of a remnant magnetic field due to the eddy current by applying the pre-emphasis to a waveform of a gradient magnetic field on the basis that a time constant in attenuation and an intensity of the eddy current are spatially uniform. Therefore, when a time constant in attenuation and an intensity of an eddy current vary spatially, a remnant magnetic field will be generated at positions where eddy currents different from a set value for the pre-emphasis are generated.
Specifically, an intensity of a remnant magnetic field, which should be corrected by applying the pre-emphasis, varies linearly in a single slice. However, an intensity of a remnant magnetic field becomes larger at a position more distant from the center of static magnetic field. Therefore, a gradient of remnant magnetic field intensity differs between slice positions. A change in a slice direction of a gradient of remnant magnetic field intensity becomes two-dimensional one and above.
Assuming that the above described change in gradient with a second order and above according to a slice position is referred to a second order and above of spatial distribution, a magnetic field generated by an eddy current has a second order of spatial distribution as well as a zero order and a first order of spatial distributions. Therefore, the effects of the zero order and the first order spatial distributions of a remnant magnetic field can be corrected by the pre-emphasis described above. However, it is difficult to correct the effects of the second order and above of spatial distributions completely by the pre-emphasis.
When the foregoing remnant magnetic field having a second order of spatial distribution due to an eddy current exists, fat tissue that has the resonance frequency different from a bandwidth of a fat saturation pulse is generated by the remnant magnetic field and the fat suppression effect is reduced. Especially, it has become a problem that the uniformity of a magnetic field at an off center is deteriorated by the effect of an eddy current having a second order of distribution and the fat suppression effect is reduced.
Therefore, in an imaging using a fat suppression technique such as the CHESS method, it is necessary to generate a uniform magnetic field to acquire an improved fat suppression effect. For that purpose, it is necessary to correct a remnant magnetic field, of which a time constant in attenuation and an intensity vary spatially, caused by an eddy current.
Similarly, when a pre-pulse is applied with a gradient magnetic field pulse having a predetermined intensity for a purpose other than the fat suppression, a gradient magnetic field with an intensity different from an original intensity of a gradient magnetic field to be generated at a timing of application of the pre-pulse is generated in case where a remnant magnetic field having a second order of spatial distribution due to an eddy current exists. Therefore, a remnant magnetic field due to an eddy current leads to degradation in accuracy of a contrast control by application of a pre-pulse. Accordingly, it is necessary to correct a remnant magnetic field caused by an eddy current at a timing of application of a pre-pulse to control a contrast.